I have been having an issue recently with my File Explorer in Windows 10 opening painfully slow, so slow that it indicated Not Responding in the status bar. I thought this was just happening on my work computer, which has an SSD, plenty of RAM and a decent CPU too. Then I discovered the same issue on my home PC which has 32 GB of RAM and realized that this must be an issue with a recent Windows update.
After some experimentation, I determined that the bug is with the Quick Launch function in File Explorer. If you set File Explorer to go to This PC or turn off the Quick Launch function, the problem stops immediately.
Open File Explorer, go to File, then select Change folder and search options.
Select This PC from Open File Explorer to field. Or you can deselect the Privacy check boxes Show recently used files in Quick access and Show frequently used folders in Quick access. Either way will get you to the solution you want.
You File Explorer should open very quickly now!